ABSTRACT The bone marrow (BM) is a heterogeneous organ with a central function in cancer progression and metastasis. It constitutes a niche for disseminated tumor cells, protecting them from therapy, promoting their dormancy and allowing them to metastasize to other organs. Cancer cells home to and hijack the BM niche converting it to a malignant niche favorable to tumor cell proliferation and survival. Thus, studying the cross-talk between tumor cells and the BM microenvironment is a subject of high interest. A major effort of our laboratory has been to study the communication between cancer cells and BM-derived mesenchymal stromal cells (BM-MSC) that form the BM-niche and that we have shown to contribute to a protumorigenic tumor microenvironment (TME). We focus on neuroblastoma (NB), a neural crest-derived tumor that is the second most common solid tumor in children, and that frequently and specifically metastasizes to the bone and BM and on tumor exosomes. These extracellular vesicles (EV) have emerged as a new and powerful mechanism of communication between tumor cells and their environment through their ability to convey multi-molecular biological messages of a much higher complexity than single growth factors. Preliminary data in this application demonstrate that NB cells release exosomes enriched in syntenin, ALIX, the tetraspanin CD-63 and Gal-3BP which are captured by BM- MSC and a pros-tumorigenic inflammatory reaction. Our overarching hypothesis is that as a result of their activity on MSC, NB-derived exosomes prime the BM niche, promoting the homing and survival of NB cells in the BM. We specifically hypothesize that syntenin plays a central role in their biogenesis and that Gal-3BP controls their uptake by interacting with a protein at the surface of BM-MSC. In a first aim we will determine the contribution of NB-derived exosomes to the pre-metastatic BM niche and the contribution of syntenin to their biogenesis, combining pharmacological and genetic (knock down and knock out) approaches in vitro in cultures of patient-derived MSC and in vivo in metastatic and non-metastatic NB cell lines. In aim 2, we will identify in BM-MSC the protein(s) interacting with Gal-3BP that contribute(s) to the uptake of exosomes in MSC, and in collaboration with Dr. David Lyden (Cornell University, NY), compare exosomes from NB and breast cancer cell lines that differ in BM tropism for their uptake by BM-MSC and BM hematopoietic stem cells (HSC) focusing on Gal-3BP and integrin in their uptake. These studies will provide novel insight into mechanisms by which exosomes are involved in the communication between tumor cells and the BM niche. It is anticipated that these studies will ultimately identify targets for intervention or prevention of cancer metastasis.